The source of organismal diversity and the link between genotype and phenotype are key questions for understanding evolution. The evolutionary mechanisms that shape phenotypes depend on population size, the complexity of genetic networks underlying traits, and the relative role of genetics versus the environment in shaping traits. In this proposal, we examine the genetic basis of opsin gene expression, which determines visual sensitivities. We choose African cichlid fishes as they have significant variation in visual sensitivity. They also have small population sizes, comparable to humans, and will help identify the types of evolutionary mechanisms that might be important in small populations. African cichlid fishes have seven cone opsin genes, with species differing in which subsets of the genes are expressed. Typically, genes are expressed in three combinations, which we call the short, medium, and long visual palettes. To some degree, the optimal palette depends on the foraging style and the water clarity where species live. For the proposed work, we have made three crosses between species that differ in opsin palette: short x medium, short x long and medium x long. These three crosses will enable us to identify the key transcription factors that shift opsin expression between these genetically determined palettes. In addition, we will study two crosses that differ in whether environmental factors, such as light spectrum, contribute to opsin expression variation. By identifying factors which control both differences between the palettes and environmental plasticity, we can determine whether these two kinds of factors are the same or different. Further, we can use RNAseq from retinal and other transcriptomes to calculate the genetic network controlling opsin expression and see how these two types of factors are distributed in that network. We predict that factors controlling fixed genetic differences will be more central to the network, while factors involved in environmental responses will be more peripheral.